Many applications that involve signal transmission (e.g., radio frequency (RF) and microwave signals) require some means of power measurement and control for the transmitted power of the signals. For example, a mobile handset terminal (e.g., a mobile phone) requires power monitoring and control to ensure compliance with regulations imposed on RF emissions and output power. GSM (Global System for Mobile communication) systems have strict power ramping requirements. CDMA (code-division multiple access) systems may implement power detection to conserve current and prolong talk time.
Typically, power measurement and control for a mobile handset device is performed using a power control loop, such as that shown in FIG. 1. An output power source 102 (e.g., a power amplifier or power amplifier module) is used to increase the power level of a signal to a required power level and deliver the signal to a load 108 (e.g., an antenna). A coupler 104 is used in the power control loop to sample a portion of the output power delivered to the load 108 and deliver the sampled output power to a detector circuit 106 to measure the output power of the signal. Key parameters of a coupler are typically insertion loss, coupling factor, size and directivity (or isolation).
In the prior art, a simple coupler may be made by using a capacitor and resistor to tap off the voltage waveform of the signal at the output of the output power source. Such a coupler provides very low insertion loss but almost no directivity. Thus any reflection at the output of the coupler 104 from the load 108 will affect the power measurement. One solution to the problem of reflection employs an isolator between the output power source 102 and the load 108. However, the use of an isolator may result in greater power loss as well as greater cost.
Systems that do not employ an isolator, such as GSM systems, require high directivity, or isolation, from reflected signals. In GSM systems, the impedance mismatch due to various antenna load conditions may result in as much as a 10:1 Voltage Standing Wave Ratio (VSWR). In this type of system, a simple one-capacitor coupler as described above can be in error by a factor of ten. For these reasons, most GSM mobile handset terminals employ elaborate distributed or lumped element couplers that have at least 10 dB of directivity. These lumped or distributed element directive couplers are typically both large and costly.
Prior art directive couplers provide an improvement in measurement accuracy over non-directive couplers in systems with variable loads, however, such couplers still produce errors in power measurement. A typical directive coupler samples a portion of an RF signal and delivers the sampled portion to a sample impedance (e.g., 50 ohms). This measurement is accurate, however, only if the load impedance is the same as the sample impedance. When the load impedance varies, errors occur in the power measurement